Method for Calculating Power Flow Solution of a Power Transmission Network that Includes Interline Power Flow Controller (IPFC)

ABSTRACT

Power flow models of Interline Power Flow Controllers (IPFC) for large-scale power systems are studied, in details. Mathematical models of the IPFC, using the d-q axis decompositions of control parameters are derived. In this framework, for each IPFC, only two control parameters are added to the unknown vector in the iteration formula and the quadratic convergence characteristic is preserved. Simulations results from several practical large-scale power systems embedded with multiple Convertible Static Compensators (CSCs) demonstrate the effectiveness of the proposed models. Comparisons with existing models are made to elucidate the performance of the convergence.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a steady-state model of Interline Power Flow Controller (IPFC) in this technical field, and more particularly to an improved structure which can be applied to simulation software of power system. Not only does this approach deliver a unique feature of rapid convergence, but it also considers the actual loss of efficiency relating to power system. So, it serves as an important foundation for installation and control of Interline Power Flow Controller (LPFC), while expanding and facilitating the congestion management/prevention of power system.

2. Description of Related Art

The newly-developed flexible AC transmission system is composed of VSCs (voltage source-based converters), a typical example of which is a Convertible Static Compensator (CSC) of maximum capacity. CSC can be configured into a few flexible AC transmission system units, e.g. Static Synchronous Compensator (STATCOM), Static Synchronous Compensator (SSSC), Unified Power Flow Controller (UPFC) and Interline Power Flow Controller (IPFC).

Of which, many scholars put more efforts on the study of steady-state models of IPFC relating to calculation of power flow, among whom Gyugyi, L. was the first one to put forward a static model of IPFC in ‘Apparatus and Method for Interline Power Flow Control’ (U.S. Pat. No. 5,698,969, Dec. 16, 1997).

Moreover, IPFC model based on Voltage Source Converter (VSC) was initiated in 1999 by Gyugyi, L., Sen, K. and Schauder, C.: ‘The Interline Power Flow Controller Concept: a New Approach to Power Flow Management in Transmission Systems’, (IEEE Trans. on Power Delivery, Vol. 3, No. 14, 1999, pp. 1115-1123). This can enable power system to operate more stably by adjusting active power/reactive power flow of transmission line in the case of congestion.

Furthermore, a new IPFC model was developed in 2003 by X.-P. Zhang, “Modeling of the Interline Power Flow Controller and the Generalized Unified Power Flow Controller in Newton Power Flow”, (IEE Proceedings. Generation, Transmission & Distribution, Vol. 150, No. 3, May. 2003, pp. 268-274). At the same time, Newton-Raphson (NR) algorithm was used to study the characteristics of IPFC model.

Subsequently, IPFC model based on VSC was further developed by B. Fardansh: ‘Optimal Utilization, Sizing, and Steady-State Performance Comparison of Multi-level VSC-Based FACTS Controller’ (IEEE Trans. on Power Delivery, Vol. 19, No. 3, July. 2004, pp. 1321-1327)—and Xuan. Wei, J. H. Chow, Behurz Fardanesh and Abdel-Aty Edris: ‘A Common Modeling Framework of Load Flow, Sensitivity, and Dispatch Analysis’ (IEEE Trans. on Power System, Vol. 19, No. 2, May. 2004, pp. 934-941).

In fact, Interline Power Flow Controller (IPFC) is based on a framework wherein Voltage Source Converters (VSCs) are linked to a DC coupling capacitor. Among them, one converter is a system with one degree of freedom, which is able to adjust the active power of transmission line, whereas the remaining converters are a system with two degree of freedoms, which are able to adjust simultaneously actual and reactive power of many transmission lines.

Owing to an increasing electrical load, existing power transmission system cannot satisfy the demand of long-distance and high-capacity power transmission. And, erection of new transmission lines remains limited for environmental protection purpose. Therefore, an important approach to resolve this problem would be to tap the potential of existing power distribution network by improving the power distribution capability. Additionally, as power systems operate in a more complex environment with the adoption of market-oriented power management system, the power system must have a stronger control ability to meet the technical and economical requirements of customers.

SUMMARY OF THE INVENTION

Thus, the first objective of the present invention is to provide a method of setting-up steady-state model of Interline Power Flow Controller (IPFC). The hybrid model of IPFC can take into account of the loss arising from coupling transformer and converter, without compromise of the convergence feature for system solution, while no initial set value for other proven technologies is required, thus exhibiting the advantages of new model such as robustness and rapid convergence.

Still, the second objective of the present invention is to provide a method of setting-up steady-state model of IPFC, which can adjust the active/reactive power of many transmission lines according to different demand-driven control objects, thereby avoiding efficiently the congestion of system and delivering a good feature of quadratic convergence.

Alternatively, the third objective of the present invention is to provide a method of setting-up steady-state model of IPFC, which can be integrated into different systems to test the power flow. It also offers better simulation results than traditional IPFC model, and presents better features of convergence and robustness.

To this end, the present invention is to provide a method of setting-up steady-state model of IPFC, which can fully depict the power conversion and flow control among IPFCs, and take into account of the loss arising from coupling transformer and converter. Interline Power Flow Controller (IPFC) comprises VSCs and a DC coupling capacitor, where all synchronous converters are linked to DC coupling capacitor. Among them, one converter is a system with one degree of freedom, which is able to adjust the reactive power of transmission line, whereas the remaining converters are a system with two degree of freedoms, which are able to adjust simultaneously actual and reactive power of many transmission lines, thus avoiding efficiently the congestion of system. When Newton-Raphson iteration method is used to calculate unknown control variables, the model of power flow controller is expressed as a d-q axis component via Park Transformation using orthogonal projection technology. This can maintain the unique feature of rapid convergence and reduce both the complexity of computational analysis and required amount of iteration variables with introduction of IPFC, without the need of additional computation.

The other features and advantages of the present invention will be more readily understood upon a thoughtful deliberation of the following detailed description of a preferred embodiment of the present invention with reference to the accompanying drawings and icons. However, it should be appreciated that the present invention is capable of a variety of embodiments and various modifications by those skilled in the art, and all such variations or changes shall be embraced within the scope of the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a framework of Interline Power Flow Controller (IPFC) of the present invention.

FIG. 2 shows an equivalent circuit of Interline Power Flow Controller (IPFC) of the present invention.

FIG. 3 shows the flow chart of calculating power flow with introduction of IPFC model.

FIG. 4 depicts the power mismatch of bus terminal voltage of Interline Power Flow Controller (IPFC) of the present invention.

FIG. 5 depicts quadratic convergence feature of systems fitted with Interline Power Flow Controller (IPFC).

FIG. 6 Convergence characteristics of control parameters of IPFC. Appendix 1 refers to Table 1 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention intends to provide a method of setting-up steady-state model of IPFC, which can fully depict the reactive power compensation and power flow control among IPFC, and take into account of the loss arising from coupling transformer and converter. Interline Power Flow Controller (IPFC) comprises VSCs and a DC coupling capacitor, where all synchronous converters are linked to DC coupling capacitor. Among them, one converter is a system with one degree of freedom, which is able to adjust the reactive power of transmission line, whereas the remaining converters are a system with two degree of freedoms, which are able to adjust simultaneously actual and reactive power of many transmission lines, thus avoiding efficiently the congestion of system. When Newton-Raphson iteration method is used to calculate unknown control variables, the model of power flow controller is expressed as a d-q axis component via Park Transformation using orthogonal projection technology. This can maintain the unique feature of rapid convergence and reduce both the complexity of computational analysis and required amount of iteration variables with introduction of IPFC, without the need of additional computation.

IPFC Static Model

Operating Principle of IPFC

The framework of Interline Power Flow Controller (IPFC) is shown in FIG. 1, wherein it comprises VSCs and a DC coupling capacitor. VSC1 is a system with one degree of freedom, which can compensate reactive power to the transmission line by a coupling transformer, and then compensate active power to VSC2˜VSCn by modulating DC coupling capacitor. VSC2˜VSCn converters are control system with one degree of freedom, which are linked to DC coupling capacitor by Back-to-Back method. All converters are linked to the transmission line via coupling transformers; each of transmission line is separately compensated with reactive power. In addition, active power is transferred to the transmission line via DC coupling capacitor. VSC2˜VSCn converters of present invention can simultaneously compensate and adjust active/reactive power, while various transmission lines are controlled independently of each other, thereby avoiding efficiently the congestion of system.

IPFC Equivalent Circuit

For the steady-state model of IPFC of the present invention, the variables are divided into two orthogonal vectors, a direct-axis and a quadrature-axis component, which controls separately converter's current phasor/d-q axis projection component, thus ensuring that the voltage of bus and active/reactive power of transmission line are subjected to decoupling control. The new model adjusts active power of transmission line using synchronous converter d-axis current, and make q-axis current responsible for adjusting reactive power. Meanwhile, parallel converter d-axis current is responsible for adjusting the voltage of DC coupling capacitor, and q-axis current for adjusting the voltage of bus at sending end. It shall be possible to judge d-q component of required variables using orthogonal projection technology. The bus voltage at sending end after d-q decomposition is expressed as: V_(xk) ^(D)+jV_(kx) ^(Q)=V_(xk)e^(j(θ) ^(xk) ^(+θ) ^(s1) ⁾   (1)

Where, upper “D” and “Q” refer to d component and q component of required variables; lower “k” refers to converter of No. k; and lower “x” can be replaced by “s”, “r”, “ser” or “sh”, indicating the variables of bus at sending end and receiving end.

Based on d-q coordinate conversion, the present invention provides a static model of Interline Power Flow Controller (IPFC). Equivalent circuit of static model of IPFC is shown in FIG. 2, where IPFC's synchronous branch represents a voltage source and equivalent impedance, while impedance represents a coupling transformer. If assuming that the loss of converter and transmission line is ignored, the transferred active power is expressed as: $\begin{matrix} {P_{dc} = {{P_{{ser}\quad 1} + {\sum\limits_{k = 2}^{n}P_{serk}}} = 0}} & (2) \end{matrix}$

Where, Pserk=input active power of VSCk, and SSSC refers to a special version of Interline Power Flow Controller (IPFC). With a single synchronous branch, equation (2) is changed into P_(dc)=P_(ser1).

IPFC Power Flow Model

IPFC Equivalent Load Model

IPFC model of present invention is equivalent to a nonlinear load. The equivalent load capacity can be modified for any iteration according to control object and voltage of bus terminal during calculation. Based on d-q coordinate conversion, the first synchronous branch current of IPFC model is expressed as: $\begin{matrix} {\begin{bmatrix} I_{{ser}\quad 1}^{D} \\ I_{{ser}\quad 1}^{Q} \end{bmatrix} = {{\frac{1}{R_{{ser}\quad 1}^{2} + X_{{ser}\quad 1}^{2}}\begin{bmatrix} R_{{ser}\quad 1} & {- X_{{ser}\quad 1}} \\ X_{{ser}\quad 1} & R_{{ser}\quad 1} \end{bmatrix}}\begin{bmatrix} {V_{s\quad 1} + V_{{ser}\quad 1}^{D} - V_{r\quad 1}^{D}} \\ {V_{{ser}\quad 1}^{Q} - V_{r\quad 1}^{Q}} \end{bmatrix}}} & (3) \end{matrix}$

Terminal voltage of IPFC in equation (2) can be obtained from equation (1). d-q coordinate axis voltages V_(ser1) ^(D) and V_(ser1) ^(Q) of first branch are unknown variables when using Newton-Raphson(N-R) iteration method, which may vary with the increase of iteration times;

According to the definition of Complex Power, the load model of first branch of IPFC is expressed as: $\begin{matrix} {\begin{bmatrix} p_{s\quad 1} \\ Q_{s\quad 1} \end{bmatrix} = {- {\begin{bmatrix} V_{s1} & 0 \\ 0 & {- V_{s1}} \end{bmatrix}\begin{bmatrix} I_{{ser}\quad 1}^{D} \\ I_{{ser}\quad 1}^{Q\quad} \end{bmatrix}}}} & (4) \\ {\begin{bmatrix} p_{r\quad 1} \\ Q_{r\quad 1} \end{bmatrix} = {- {\begin{bmatrix} V_{r\quad 1}^{Q} & V_{r\quad 1}^{Q} \\ V_{r\quad 1}^{Q} & {- V_{r\quad 1}^{Q}} \end{bmatrix}\begin{bmatrix} I_{{ser}\quad 1}^{D} \\ I_{{ser}\quad 1}^{Q} \end{bmatrix}}}} & (5) \end{matrix}$

Apart from the first branch, other branches of IPFC control the objects according to differnt power flows, with the equivalent load models expressed as: $\begin{matrix} \begin{matrix} {{\begin{bmatrix} p_{rk} \\ Q_{rk} \end{bmatrix} = {- \begin{bmatrix} P_{linek}^{ref} \\ Q_{linek}^{ref} \end{bmatrix}}},} & {{k = 2},\Lambda,n} \end{matrix} & (6) \end{matrix}$

Where, P_(linek) ^(ref) and Q_(linek) ^(ref) are reference values of active and reactive power of bus at receiving end of No. k branch circuit. Apart from the first branch, the equivalent load models of other branches of IPFC feeding the bus are: $\begin{matrix} {\begin{bmatrix} p_{sk} \\ Q_{sk} \end{bmatrix} = {- {\begin{bmatrix} V_{sk}^{D} & V_{sk}^{Q} \\ V_{sk}^{Q} & {- V_{sk}^{D}} \end{bmatrix}\begin{bmatrix} I_{serk}^{D} \\ I_{serk}^{Q} \end{bmatrix}}}} & (7) \end{matrix}$

Where, $\begin{bmatrix} I_{serk}^{D} \\ I_{serk}^{Q} \end{bmatrix} = {- {{\frac{1}{V_{rk}^{2}}\begin{bmatrix} V_{rk}^{D} & V_{rk}^{Q} \\ V_{rk}^{Q} & {- V_{rk}^{D}} \end{bmatrix}}\begin{bmatrix} P_{rk} \\ Q_{rk} \end{bmatrix}}}$ Power Compensation of Converter

VSC1 and VSC2-VSCn of Interline Power Flow Controller (IPFC) are available with different functions, by which input power of VSCs after algebraic operation are: P_(ser1)=I_(ser1) ^(D)V_(ser1) ^(D)+I_(ser1) ^(Q)V_(ser1) ^(Q)   (8) P_(serk)=I_(serk) (V_(rk) ^(D)−V_(sk) ^(D))+I_(serk) ^(Q)(V_(rk) ^(D)+V_(sk) ^(Q))+I_(serk) ^(D) ² +I_(serk) ^(Q) ² )R_(serk)   (9)

VSC1 is used to maintain a balanced active power among converters, with its power exchange conducted through transmission line of VSC2-VSCn. In addition, VSC1 provides a compensation of reactive power, and controls the active/reactive power between sedning end s1 and receiving end r1 of the bus: f _(ser1)=P_(r1)+P_(line1) ^(ref)=0 or f _(ser1)=Q_(r1)+Q_(line 1) ^(ref)   (10) N-R Iteration Algorithm

Power flow equation is obtained from N-R method, with the iteration equation illustrated below: x ^((k+1)) =x ^((k))+J⁻¹f(x)   (11)

Where, x=unknown vectors. The variables include voltage and phase angle of bus as well as independent variable of CSC. f(x) refers to mismatch vector of actual and reactive power of buss, J refers to corresponding Jacobian matrix. Therefore, IPFC model will be replaced by two nonlinear equivalent loads, into which mismatch vector is incorporated. Interline Power Flow Controller (IPFC) model is expressed again as: f′=f+Δf_(IPFC)   (12)

Where, $\begin{matrix} {{\Delta\quad f_{IPFC}} = \left\lbrack {\Delta\quad f_{bus}}\quad \middle| \quad{\Delta\quad f_{control}} \right\rbrack^{T}} \\ {= \left\lbrack {P_{s\quad 1}\quad Q_{s\quad 1}\quad P_{r\quad 1}\quad Q_{r\quad 1}\quad P_{sk}\quad Q_{sk}\quad P_{rk}\quad Q_{rk}}\quad \middle| \quad{P_{dc}\quad f_{{ser}\quad 1}} \right\rbrack^{T}} \end{matrix}$

Where, f′ considers the mismatch vector of equivalent load of Interline Power Flow Controller (IPFC), Δf_(IPFC) includes Δf_(Bus) and Δf_(Control), of which Δf_(Bus) refers to the variable of bus terminal of Interline Power Flow Controller (IPFC), and Δf_(Control) refers to control variable related to Interline Power Flow Controller (IPFC).

Unknown vectors will vary from different iteration equations. In Interline Power Flow Controller (IPFC), V_(ser1) ^(D) and V_(Ser1) ^(Q) indicate status variables. Thus, the unknown vector elements of Interline Power Flow Controller (IPFC) model can be expressed as: $\begin{matrix} \begin{matrix} {x_{IPFC} = \left\lbrack x_{bus}\quad \middle| \quad x_{Control} \right\rbrack^{T}} \\ {= \left\lbrack {\theta_{s\quad 1}\quad V_{s\quad 1}\quad\theta_{r\quad 1}\quad V_{r\quad 1}\quad\theta_{sk}\quad V_{sk}\quad\theta_{rk}\quad V_{rk}}\quad \middle| \quad{V_{{ser}\quad 1}^{D}\quad V_{{ser}\quad 1}^{Q}} \right\rbrack^{T}} \end{matrix} & (13) \end{matrix}$

Where, x_(Bus) is represented by original status variable and x_(Control) by new control variable. Jacobian matrix of IPFC can be obtained from one-order partial differentiation equation for f′: J′=J+ΔJ_(IPFC)   (14)

Where: ${\Delta\quad J_{IPFC}} = \begin{bmatrix} \frac{\partial P_{s\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial P_{s\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial P_{s\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial P_{s\quad 1}}{\partial V_{r\quad 1}} & 0 & 0 & 0 & 0 & | & \frac{\partial P_{s\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial P_{s\quad 1}}{\partial V_{{ser}\quad 1}^{Q}} \\ \frac{\partial Q_{s\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial Q_{s\quad 1}}{\partial V_{s\quad 1}} & \frac{\partial Q_{s\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial Q_{s\quad 1}}{\partial V_{r\quad 1}} & 0 & 0 & 0 & 0 & | & \frac{\partial Q_{s\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial Q_{s\quad 1}}{\partial V_{{ser}\quad 1}^{D}} \\ \frac{\partial P_{r\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial P_{r\quad 1}}{\partial V_{s\quad 1}} & \frac{\partial P_{r\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial P_{r\quad 1}}{\partial V_{r\quad 1}} & 0 & 0 & 0 & 0 & | & \frac{\partial P_{r\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial P_{r\quad 1}}{\partial V_{{ser}\quad 1}^{D}} \\ \frac{\partial Q_{r\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial Q_{r\quad 1}}{\partial V_{s\quad 1}} & \frac{\partial Q_{r\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial P_{s\quad 1}}{\partial V_{s\quad 1}} & 0 & 0 & 0 & 0 & | & \frac{\partial Q_{r\quad 1}}{\partial V_{{ser}\quad 1}^{D\quad}} & \frac{\partial Q_{r\quad 1}}{\partial V_{{ser}\quad 1}^{D}} \\ \frac{\partial P_{sk}}{\partial\theta_{s\quad 1}} & 0 & 0 & 0 & \frac{\partial P_{sk}}{\partial\theta_{sk}} & \frac{\partial P_{sk}}{\partial V_{sk}} & \frac{\partial P_{sk}}{\partial\theta_{rk}} & \frac{\partial P_{sk}}{\partial V_{rk}} & | & 0 & 0 \\ \frac{\partial Q_{sk}}{\partial\theta_{s\quad 1}} & 0 & 0 & 0 & \frac{\partial Q_{sk}}{\partial\theta_{sk}} & \frac{\partial Q_{sk}}{\partial V_{sk}} & \frac{\partial Q_{sk}}{\partial\theta_{rk}} & \frac{\partial Q_{sk}}{\partial V_{rk}} & | & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & | & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & | & 0 & 0 \\  - & - & - & - & - & - & - & - & + & - & - \\ \frac{\partial P_{dc}}{\partial\theta_{s\quad 1}} & \frac{\partial P_{dc}}{\partial V_{s\quad 1}} & \frac{\partial P_{dc}}{\partial\theta_{r\quad 1}} & \frac{\partial P_{dc}}{\partial V_{r\quad 1}} & \frac{\partial P_{dc}}{\partial\theta_{sk}} & \frac{\partial P_{dc}}{\partial V_{sk}} & \frac{\partial P_{dc}}{\partial\theta_{rk}} & \frac{\partial P_{dc}}{\partial V_{rk}} & | & \frac{\partial P_{dc}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial P_{dc}}{\partial V_{{ser}\quad 1}^{Q}} \\ \frac{\partial f_{{ser}\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial f_{{ser}\quad 1}}{\partial V_{s\quad 1}} & \frac{\partial f_{{ser}\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial f_{{ser}\quad 1}}{\partial V_{r\quad 1}} & 0 & 0 & 0 & 0 & | & \frac{\partial f_{{ser}\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial f_{{ser}\quad 1}}{\partial V_{{ser}\quad 1}^{Q}} \end{bmatrix}$

P_(rk) and Q_(rk) in matrix ΔJ_(IPFC) are constants, while both column 7 and 8 are referred to zero, and the elements at upper left corner mean an original Jocobin matrix. IPFC of present invention will have ΔJ_(IPFC) increased by two orders. Accordingly, unknown vector and mismatch vector will increase by two orders, indicating that, when IPFC parameters are added to Jacobian matrix, this matrix can only increase by two orders as compared to traditional model. But, it can facilitate its calculation convergence speed and maintain original quadratic convergence feature. This theoretical derivation will be verified by subsequent simulation results.

Model of Converter

If flow solution of Unified Power Flow Controller (UPFC) model can be converged, d-q component of VSC2-VSCn voltage may be expressed as: $\begin{matrix} \begin{matrix} {{\begin{bmatrix} V_{serk}^{D} \\ V_{serk}^{Q} \end{bmatrix} = {{\begin{bmatrix} R_{serk} & {- X_{serk}} \\ X_{serk} & R_{serk} \end{bmatrix}\begin{bmatrix} I_{serk}^{D} \\ I_{serk}^{Q} \end{bmatrix}} + \begin{bmatrix} {V_{rk}^{D} - V_{sk}^{D}} \\ {V_{rk}^{Q} - V_{sk}^{Q}} \end{bmatrix}}},} & {{k = 2},\Lambda,n} \end{matrix} & (15) \end{matrix}$

Thus, the size and phase of synchronous voltage of VSC2-VSCn can be expressed as: $\begin{matrix} {V_{serk} = {{V_{serk}{\angle\theta}_{serk}} = {\sqrt{V_{serk}^{D^{2}} + V_{serk}^{D^{2\quad}}}{\angle\left( {{\tan^{- 1}\frac{V_{serk}^{Q}}{V_{serk}^{D}}} + \theta_{s\quad 1}} \right)}}}} & (16) \end{matrix}$

Case Analysis

To verify the applicability of IPFC model of the present invention, it shall be mounted into different test systems. FIG. 3 shows a flow chart of calculating power flow with introduction of IPFC model. The first step (301) is to calculate mismatch vector, then establish Jacobian matrix in step (302). Next, step (303) is to obtain d-q component of bus voltage V_(r) ^(D) and V_(r) ^(Q) at receiving end after using Park Transformation, and step (304) to calculate actual and reactive power of bus at VSC1 sending end and receiving end. Furthermore, step (305) is to calculate active and reactive power of bus at VSC2-VSCn sending end and receiving end, step (306) to calculate active power Pdc of coupling capacitor, followed by steps (307) and (308) to modify mismatch vector and Jacobian matrix, and step (309) to amend unknown vector via N-R iteration method, step (310) to judge the convergence of flow solution. Otherwise, return to step (301) to recalculate mismatch vector. In the case of convergence, the final step (311) is to obtain the voltage of converter. In the end, static model of IPFC of present invention and GUPFC of CSC family are added into two test systems, whereby Matpower 2.0 is used to verify the performance. The test system includes IEEE 57 bus system and IEEE 118 bus system, which conduct analysis in the following four cases:

-   -   1. Case A: analyze IEEE 57 bus system, without installation of         any Interline Power Flow Controller (IPFC).     -   2. Case B: IEEE 57 bus system is fitted with an Interline Power         Flow Controller (IPFC) and GUPFC. Interline Power Flow         Controller (IPFC), installed between transmission line 8-7 and         9-13, is able to control active power of transmission line 8-7         and active/reactive power of transmission line 9-13. GUPFC is         able to control both the voltage of bus and active/reactive         power flow of transmission line 56-42 and 41-11.     -   3. Case C: analyze IEEE 118 bus system, without installation of         Interline Power Flow Controller (IPFC) and GUPFC.     -   4. Case D: IEEE 57 bus system is fitted with two 2 Interline         Power Flow Controller (IPFC) and 2 GUPFC. IPFC1, installed         between transmission line 12-11 and 12-3, is able to control         active power of transmission line 12-11 and active/reactive         power of transmission line 12-3. IPFC2, installed between         transmission line 80-77 and 80-97transmission line, is able to         control active power of transmission line 80-77 and         actual/reactive power of transmission line 80-97. GUPFC1 is able         to control both the voltage of bus 45 and active/reactive power         flow of transmission line 45-44 and 45-46. GUPFC2 is able to         control both the voltage of bus 94 and active/reactive power         flow of transmission line 94-95, 94-93 and 94-100.

It is assumed that all parameters of coupling transformer are: Rser=0.01 p.u. and Xser=0.1 p.u. . . . maximum permissible tolerance of iteration is 10-12, and initial control parameters V_(ser1) ^(D) and V_(ser2) ^(Q) of Interline Power Flow Controller (IPFC) have a set value of zero. The comparison of iteration times required for system convergence in different cases is listed in Table 1(e.g. FIG. 7). The simulation results show that, the system can improve its stability and maintain an excellent convergence feature with introduction of IPFC model.

To verify the applicability of model initiated by the present invention, Interline Power Flow Controller (IPFC) is linked to different busses. FIG. 4 shows the power mismatch of bus terminal voltage with introduction of Interline Power Flow Controller (IPFC), wherein the mismatch is close to 10-15 after 5 iterations. The iteration times of test system required for power flow solution in different cases are listed in Table 1. For any test system of the same kind that's fitted with a IPFC model initiated by Zhang, X.-P., “Modeling of the Interline Power Flow Controller and the Generalized Unified Power Controller in Newton Power Flow” (IEE Proc. Gener. Trans. Distrib., Vol. 3, No. 150, pp. 268-274, 2003), it's required to obtain converged flow solution after 8 iteration calculations. But in fact, the test system fitted with IPFC of present invention can obtain converged flow solution after 6 iteration calculations, showing that IPFC of present invention features a rapid convergence. FIG. 5 depitcs the results of quadratic convergence for IPFC in Case D, where the broken line is a typical quadratic convergence curve, showing that quadratic convergence curve of present invention is close to a typical curve. FIG. 6 depicts the convergence results of IEEE 118 bus system fitted with an Interline Power Flow Controller(IPFC), which can offer a nearly optimal convergence value after 2 iterations, showing an excellent convergence feature of this sytem.

In brief, the aforementioned involve an innovative invention that can promote overall economic efficiency thanks to its many functions and actual value. And, no similar products or equivalent are applied in this technical field, so it would be appreciated that the present invention is granted patent as it meets the patent-pending requirements. 

1. (canceled)
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 6. A method for incorporating a steady-state model of Interline Power Flow Controller (IPFC) into a Newton-Raphson algorithm, said method comprising the steps of: A) calculating the original mismatch vector without considering the IPFC; B) establishing the Jacobian matrix corrsponding to the mismatch vector in step A); C) obtaining the d-q components of the receiving-end bus voltage of IPFC by a d-q axis decomposition; D) calculating the equivalent active and reactive loading at the sending-end and receiving-end buses of the first branch of IPFC; E) calculating the equivalent active and reactive loading at the sending-end and receiving-end buses of remaining series branches; F) calculating the active power flowing into the DC coupling capacitor; G) modifying the mismatch vector and Jacobian matrix; H) updating the unknown vector by an iteration formula; I) judging the convergence of the power flow solution; if the solution converges within the specified tolerance, going to step J); otherwise, going to step A); J) calculating the equivalent voltage of converter.
 7. A method in accordance with claim 6, wherein the d-q components of the receiving-end bus voltage in step (C) is calculated as following: V_(r1) ^(D)+jV_(r1) ^(Q)=|V_(r1)|e^(j(θ) ^(r1) ^(−θ) ^(s1) ⁾.
 8. A method in accordance with claim 7, wherein the equivalent active and reactive loading at the sending-end and receiving-end buses of the first branch of IPFC in step (D) is obtained as following: $\begin{bmatrix} P_{s\quad 1} \\ Q_{s\quad 1} \end{bmatrix} = {{- {{\begin{bmatrix} V_{s\quad 1} & 0 \\ 0 & {- V_{s\quad 1}} \end{bmatrix}\begin{bmatrix} I_{{ser}\quad 1}^{D} \\ I_{{ser}\quad 1}^{Q} \end{bmatrix}}\begin{bmatrix} P_{r\quad 1} \\ Q_{r\quad 1} \end{bmatrix}}} = {- {{\begin{bmatrix} V_{r\quad 1}^{D} & V_{r\quad 1}^{Q} \\ V_{r\quad 1}^{Q} & {- V_{r\quad 1}^{D}} \end{bmatrix}\begin{bmatrix} I_{{ser}\quad 1}^{D} \\ I_{{ser}\quad 1}^{Q} \end{bmatrix}}.}}}$
 9. A method in accordance with claim 8, wherein the equivalent active and reactive loading at the sending-end and receiving-end buses of remaining series branches in step (E) are obtained as following: ${\begin{bmatrix} P_{s\quad k} \\ Q_{s\quad k} \end{bmatrix} = {{- {{\begin{bmatrix} V_{sk}^{D} & V_{sk}^{Q} \\ V_{sk}^{Q} & {- V_{sk}^{D}} \end{bmatrix}\begin{bmatrix} I_{serk}^{D} \\ I_{serk}^{Q} \end{bmatrix}}\begin{bmatrix} P_{r\quad k} \\ Q_{r\quad k} \end{bmatrix}}} = {- \begin{bmatrix} P_{linek}^{ref} \\ Q_{linek}^{ref} \end{bmatrix}}}},\quad{k = 2},\Lambda,n$ ${where},{\begin{bmatrix} I_{serk}^{D} \\ I_{serk}^{Q} \end{bmatrix} = {- {{{\frac{1}{V_{rk}^{2}}\begin{bmatrix} V_{rk}^{D} & V_{rk}^{Q} \\ V_{rk}^{Q} & {- V_{rk}^{D}} \end{bmatrix}}\begin{bmatrix} P_{r\quad k} \\ Q_{r\quad k} \end{bmatrix}}.}}}$
 10. A method in accordance with claim 9, wherein the active power flowing from the DC coupling capacitor in step (F) is obtained as following: ${P_{dc} = {{P_{{ser}\quad 1} + {\sum\limits_{k = 2}^{n}P_{serk}}} = 0}},{where}$ P_(ser  1) = I_(ser  1)^(D)V_(ser  1)^(D) + I_(ser  1)^(Q)V_(ser  1)^(Q) P_(serk) = I_(serk)^(D)(V_(rk)^(D) − V_(sk)^(D)) + I_(serk)^(Q)(V_(rk)^(Q) + V_(sk)^(Q)) + (I_(serk)^(D²) + I_(serk)^(Q²))R_(serk).
 11. A method in accordance with claim 10, wherein the mismatch vector and Jacobian matrix in step (G) is modified as following: f^(′) = f + Δ  f_(IPFC); J^(′) = J + Δ  J_(IPFC);   ${where},\begin{matrix} {{{\Delta\quad f_{IPFC}} = \left\lbrack {\Delta\quad f_{bus}} \middle| {\Delta\quad f_{control}} \right\rbrack^{T}},} \\ {{= \begin{bmatrix} P_{s\quad 1} & Q_{s\quad 1} & P_{r\quad 1} & Q_{r\quad 1} & P_{sk} & Q_{sk} & P_{rk} & Q_{rk} & | & P_{dc} & f_{{ser}\quad 1} \end{bmatrix}^{T}},} \end{matrix}$ ${\Delta\quad J_{IPFC}} = {\begin{bmatrix} \frac{\partial P_{s\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial P_{s\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial P_{s\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial P_{s\quad 1}}{\partial V_{r\quad 1}} & 0 & 0 & 0 & 0 & \text{|} & \frac{\partial P_{s\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial P_{s\quad 1}}{\partial V_{{ser}\quad 1}^{D}} \\ \frac{\partial Q_{s\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial Q_{s\quad 1}}{\partial V_{s\quad 1}} & \frac{\partial Q_{s\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial Q_{s\quad 1}}{\partial V_{r\quad 1}} & 0 & 0 & 0 & 0 & \text{|} & \frac{\partial Q_{s\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial Q_{s\quad 1}}{\partial V_{{ser}\quad 1}^{D}} \\ \frac{\partial P_{r\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial P_{r\quad 1}}{\partial V_{s\quad 1}} & \frac{\partial P_{r\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial P_{r\quad 1}}{\partial V_{r\quad 1}} & 0 & 0 & 0 & 0 & \text{|} & \frac{\partial P_{r\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial P_{r\quad 1}}{\partial V_{{ser}\quad 1}^{D}} \\ \frac{\partial Q_{r\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial Q_{r\quad 1}}{\partial V_{s\quad 1}} & \frac{\partial Q_{r\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial P_{s\quad 1}}{\partial V_{s\quad 1}} & 0 & 0 & 0 & 0 & \text{|} & \frac{\partial Q_{r\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial Q_{r\quad 1}}{\partial V_{{ser}\quad 1}^{D}} \\ \frac{\partial P_{sk}}{\partial\theta_{s\quad 1}} & 0 & 0 & 0 & \frac{\partial P_{sk}}{\partial\theta_{sk}} & \frac{\partial P_{s\quad k}}{\partial V_{sk}} & \frac{\partial P_{s\quad k}}{\partial\theta_{rk}} & \frac{\partial P_{s\quad k}}{\partial V_{{rk}\quad}} & \text{|} & 0 & 0 \\ \frac{\partial Q_{sk}}{\partial\theta_{s\quad 1}} & 0 & 0 & 0 & \frac{\partial Q_{s\quad k}}{\partial\theta_{sk}} & \frac{\partial Q_{s\quad k}}{\partial V_{sk}} & \frac{\partial Q_{s\quad k}}{\partial\theta_{rk}} & \frac{\partial Q_{s\quad k}}{\partial V_{{rk}\quad}} & \text{|} & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & \text{|} & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & \text{|} & 0 & 0 \\  - & - & - & - & - & - & - & - & + & - & - \\ \frac{\partial P_{dc}}{\partial\theta_{s\quad 1}} & \frac{\partial P_{dc}}{\partial V_{s\quad 1}} & \frac{\partial P_{dc}}{\partial\theta_{r\quad 1}} & \frac{\partial P_{dc}}{\partial V_{r\quad 1}} & \frac{\partial P_{dc}}{\partial\theta_{s\quad k}} & \frac{\partial P_{dc}}{\partial V_{sk}} & \frac{\partial P_{dc}}{\partial\theta_{rk}} & \frac{\partial P_{dc}}{\partial\theta_{rk}} & \text{|} & \frac{\partial P_{dc}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial P_{dc}}{\partial V_{{ser}\quad 1}^{Q}} \\ \frac{\partial f_{{ser}\quad 1}}{\partial\theta_{s\quad 1}} & \frac{\partial f_{{ser}\quad 1}}{\partial V_{s\quad 1}} & \frac{\partial f_{{ser}\quad 1}}{\partial\theta_{r\quad 1}} & \frac{\partial f_{{ser}\quad 1}}{\partial V_{r\quad 1}} & 0 & 0 & 0 & 0 & \text{|} & \frac{\partial f_{{ser}\quad 1}}{\partial V_{{ser}\quad 1}^{D}} & \frac{\partial f_{{ser}\quad 1}}{\partial V_{{ser}\quad 1}^{Q}} \end{bmatrix}.}$
 12. A method in accordance with claim 11, wherein the unknown vector in step (H) is updated by a Newton-Raphson iteration formula as following: x ^((k+1)) =x ^((k))+J′⁻¹f′(x).
 13. A method in accordance with claim 12, wherein in step (J) is calculated as following: ${V_{serk} = {{V_{serk}{\angle\theta}_{serk}} = {\sqrt{V_{serk}^{D^{2}} + V_{serk}^{D^{2}}}{\angle\left( {{\tan^{- 1}\frac{V_{serk}^{Q}}{V_{serk}^{D}}} + \theta_{s\quad 1}} \right)}}}},{{{where}\begin{bmatrix} V_{serk}^{D} \\ V_{serk}^{Q} \end{bmatrix}} = {{\begin{bmatrix} R_{serk} & {- X_{serk}} \\ X_{serk} & R_{serk} \end{bmatrix}\begin{bmatrix} I_{serk}^{D} \\ I_{serk}^{Q} \end{bmatrix}} + \begin{bmatrix} {V_{rk}^{D} - V_{sk}^{D}} \\ {V_{rk}^{Q} - V_{sk}^{Q}} \end{bmatrix}}},\quad{k = 2},\Lambda,{n.}$ 